Stimulation of an immune response is dependent upon the presence of antigens recognised as foreign by the host immune system. The discovery of the existence of tumour associated antigens has now raised the possibility of using a host's immune system to intervene in tumour growth. Various mechanisms of harnessing both the humoral and cellular arms of the immune system are currently being explored for cancer immunotherapy.
Specific elements of the cellular immune response are capable of specifically recognising and destroying tumour cells. The isolation of cytotoxic T-cells (CTL) from tumour-infiltrating cell populations or from peripheral blood suggests that such cells play an important role in natural immune defences against cancer (Cheever et al., Annals N.Y. Acad. Sci. 1993 690:101-112). CD8-positive T-cells (TCD8+) in particular, which recognise Class I molecules of the major histocompatibility complex (MHC)-bearing peptides of usually 8 to 10 residues derived from proteins located in the cytosol, play an important role in this response. The MHC-molecules in humans are also designated as human leukocyte-antigens (HLA).
There are two classes of MHC-molecules: MHC-I-molecules and MHC-molecules. MHC-I molecules can be found on most cells having a nucleus, and present peptides that result from proteolytic cleavage of endogenous proteins and larger peptides. MHC-II-molecules can be found only on professional antigen presenting cells (APC), and present peptides of exogenous proteins that are taken up by APCs during the course of endocytosis, and are subsequently processed. Complexes of peptide and MHC-I molecules are recognised by CD8-positive cytotoxic T-lymphocytes, and complexes of peptide and MHC-II molecules are recognised by CD4-positive-helper-T-cells.
CD4-positive helper T-cells play an important role in orchestrating the effector functions of anti-tumor T-cell responses and for this reason the identification of CD4-positive T-cell epitopes derived from tumor associated antigens (TAA) may be of great importance for the development of pharmaceutical products for triggering anti-tumor immune responses (Kobayashi, H., R. Omiya, M. Ruiz, E. Huarte, P. Sarobe, J. J. Lasarte, M. Herraiz, B. Sangro, J. Prieto, F. Borras-Cuesta, and E. Celis. 2002. Identification of an antigenic epitope for helper T lymphocytes from carcinoembryonic antigen. Clin. Cancer Res. 8:3219-3225, Gnjatic, S., D. Atanackovic, E. Jager, M. Matsuo, A. Selvakumar, N. K. Altorki, R. G. Maki, B. Dupont, G. Ritter, Y. T. Chen, A. Knuth, and L. J. Old. 2003. Survey of naturally occurring CD4+ T-cell responses against NY-ESO-1 in cancer patients: Correlation with antibody responses. Proc. Natl. Acad. Sci. U.S.A. 100(15):8862-7).
It was shown in mammalian animal models, e.g., mice, that even in the absence of cytotoxic T lymphocyte (CTL) effector cells (i.e., CD8-positive T lymphocytes), CD4-positive T-cells are sufficient for inhibiting visualization of tumors via inhibition of angiogenesis by secretion of interferon-gamma (IFNγ) (Qin, Z. and T. Blankenstein. 2000. CD4+ T-cell-mediated tumor rejection involves inhibition of angiogenesis that is dependent on IFN gamma receptor expression by nonhematopoietic cells. Immunity. 12:677-686). Additionally, it was shown that CD4-positive T-cells recognizing peptides from tumor-associated antigens presented by HLA class II molecules can counteract tumor progression via the induction of an Antibody (Ab) responses (Kennedy, R. C., M. H. Shearer, A. M. Watts, and R. K. Bright. 2003. CD4+ T lymphocytes play a critical role in antibody production and tumor immunity against simian virus 40 large tumor antigen. Cancer Res. 63:1040-1045). In contrast to tumor-associated peptides binding to HLA class I molecules, only a small number of class II ligands of TAA have been described so far (www.cancerimmunity.org, www.syfpeithi.de). Since the constitutive expression of HLA class II molecules is usually limited to cells of the immune system (Mach, B., V. Steimle, E. Martinez-Soria, and W. Reith. 1996. Regulation of MHC class II genes: lessons from a disease. Annu. Rev. Immunol. 14:301-331), the possibility of isolating class II peptides directly from primary tumors was not considered possible. Therefore, numerous strategies to target antigens into the class II processing pathway of antigen presenting cells (APCs) have been described. For example, the APCs having been incubated with the antigen of interest to enable it to be taken up, processed and presented (Chaux, P., V. Vantomme, V. Stroobant, K. Thielemans, J. Corthals, R. Luiten, A. M. Eggermont, T. Boon, and B. P. van der Bruggen. 1999. Identification of MAGE-3 epitopes presented by HLA-DR molecules to CD4(+) T lymphocytes. J. Exp. Med. 189:767-778), or cells have been transfected with genes or minigenes encoding the antigen of interest and fused to the invariant chain, which mediates the translocation of antigens to the lysosomal MHC class II processing and assembling compartment (MIIC).
For a peptide to trigger (elicit) a cellular immune response, it must bind to an MHC-molecule. This process is dependent on the allele of the MHC-molecule and specific polymorphisms of the amino acid sequence of the peptide. MHC-class-I-binding peptides are usually 8-10 residues in length and contain two conserved residues (“anchor”) in their sequence that interact with the corresponding binding groove of the MHC-molecule.
In the absence of inflammation, expression of MHC class II molecules is mainly restricted to cells of the immune system, especially professional antigen-presenting cells (APC), e.g., monocytes, monocyte-derived cells, macrophages, dendritic cells.
The antigens that are recognised by the tumour specific cytotoxic T-lymphocytes, that is, their epitopes, can be molecules derived from all protein classes, such as enzymes, receptors, transcription factors, etc. Furthermore, tumour associated antigens, for example, can also be present in tumour cells only, for example as products of mutated genes or from alternative open reading frames (ORFs), or from protein splicing (Vigneron N, Stroobant V, Chapiro J, Ooms A, Degiovanni G, Morel S, van der Bruggen P, Boon T, Van den Eynde B J. An antigenic peptide produced by peptide splicing in the proteasome. Science. 2004 Apr. 23; 304 (5670):587-90.). Another important class of tumour associated antigens are tissue-specific structures, such as CT (“cancer testis”)-antigens that are expressed in different kinds of tumours and in healthy tissue of the testis.
Various tumour associated antigens have been identified. Further, much research effort is being expended to identify additional tumour associated antigens. Some groups of tumour associated antigens, also referred to in the art as tumour specific antigens, are tissue specific. Examples include, but are not limited to, tyrosinase for melanoma, PSA and PSMA for prostate cancer and chromosomal cross-overs such as bcr/abl in lymphoma. However, many tumour associated antigens that have been identified occur in multiple tumour types, and some, such as oncogenic proteins and/or tumour suppressor genes (tumour suppressor genes are, for example reviewed for renal cancer in Linehan W M, Walther M M, Zbar B. The genetic basis of cancer of the kidney. J Urol. 2003 December; 170(6 Pt 1):2163-72) which actually cause the transformation event, occur in nearly all tumour types. For example, normal cellular proteins that control cell growth and differentiation, such as p53 (which is an example for a tumour suppressor gene), ras, c-met, myc, pRB, VHL, and HER-2/neu, can accumulate mutations resulting in upregulation of expression of these gene products thereby making them oncogenic (McCartey et al. Cancer Research 1998 15:58 2601-5; Disis et al. Ciba Found. Symp. 1994 187:198-211). These mutant proteins can be the target of a tumour specific immune response in multiple types of cancer.
For the proteins to be recognised by the cytotoxic T-lymphocytes as tumour-specific antigen, and to be used in a therapy, particular prerequisites must be fulfilled. The antigen should be expressed mainly by tumour cells and not by normal healthy tissues or at the very least, expressed in rather small amounts in normal healthy tissue. It is furthermore desirable that the respective antigen is not only present in one type of tumour, but also in high concentrations (e.g. copy numbers per cell). The presence of epitopes in the amino acid sequence of the antigen is essential, since such peptide (“immunogenic peptide”) that is derived from a tumour associated antigen should lead to an in vitro or in vivo T-cell-response.
Until now, numerous strategies to target antigens into the class II processing pathway have been described. It is possible to incubate antigen presenting cells (APCs) with the antigen of interest to be taken up and processed (Chaux, P., Vantomme, V., Stroobant, V., Thielemans, K., Corthals, J., Luiten, R., Eggermont, A. M., Boon, T. & van der, B. P. (1999) J. Exp. Med. 189, 767-778). Other strategies use fusion proteins that contain lysosomal target sequences. Expressed in APCs, such fusion proteins direct the antigens into the class II processing compartment (Marks, M. S., Roche, P. A., van Donselaar, E., Woodruff, L., Peters, P. J. & Bonifacino, J. S. (1995) J. Cell Biol. 131, 351-369, Rodriguez, F., Harkins, S., Redwine, J. M., de Pereda, J. M. & Whitton, J. L. (2001) J. Virol. 75, 10421-10430).
T-helper cells play an important role in orchestrating the effector function of CTLs in anti-tumour immunity. T-helper cell epitopes that trigger a T-helper cell response of the TH1 type support effector functions of CD8-positive Killer T-cells, which include cytotoxic functions directed against tumour cells displaying tumour-associated peptide/MHC complexes on their cell surfaces. In this way tumour-associated T-helper cell peptide epitopes, alone or in combination with other tumour-associated peptides, can serve as active pharmaceutical ingredients of vaccine compositions which stimulate anti-tumour immune responses.
The major task in the development of a tumour vaccine is therefore the identification and characterisation of novel tumour associated antigens and immunogenic T-helper epitopes derived therefrom, that can be recognised by CD4-positive CTLs. Therefore, there is a need to provide novel amino acid sequences for peptides that have the ability to bind to a molecule of the human major histocompatibility complex (MHC) class-II. The present invention fulfils this need.